feat: Implement Prompt Builder functionality in Popup and Options

- Added a new "Prompt Builder" tab in the Popup for generating AI prompts with customizable parameters.
- Introduced new state variables for managing prompt styles, personas, formats, and models.
- Enhanced the Options page to fetch and display models based on the provided API key.
- Updated the actions and types to include the new 'prompt' action and its associated parameters.
- Implemented migration logic for legacy plaintext API keys to encrypted storage.
- Updated the getSystemPrompt function to incorporate prompt parameters for better instruction generation.
- Added tests for the new functionality, including context menu entries and prompt generation logic.

docs/LESSONS_LEARNED.md

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+# Lessons learned — LexAI
+
+> Evidence-backed invariants and guardrails for this codebase. Not a transcript or issue tracker. The one-line active rules live in `CLAUDE.md` → `## Lessons`; the detail lives here.
+
+## Active guardrails
+
+`CLAUDE.md` → `## Lessons` is the canonical active list loaded every session. The entries below are the established codebase invariants (from Phase 1 and the 2026-07-13 code read) that break things silently when violated.
+
+### L-CORE-01 — `onMessage` listener must `return true`
+
+- **Root cause / failure boundary:** async provider responses need the message channel held open; a listener that doesn't `return true` drops every response with no error.
+- **Prevention:** never remove `return true` from the `chrome.runtime.onMessage` handler in `background.ts`.
+- **Eval:** manual (candidate: a unit test asserting the listener returns `true`).
+
+### L-CORE-02 — Snapshot selection before any `await`
+
+- **Root cause / failure boundary:** focus shifts to the toolbar and the live selection is gone by the time an async response returns.
+- **Prevention:** in `content.ts`, capture active element + offsets eagerly (mouseup + button mousedown) and snapshot before awaiting; Replace uses the snapshot. Handle textarea/input (`selectionStart/End`) **and** contenteditable/DOM (`Range` API).
+- **Eval:** E-BASE-04 manual; DOM test tracked in TASKS #10.
+
+### L-CORE-03 — Keep both `ANALYZE_TEXT` message shapes
+
+- **Root cause / failure boundary:** callers send both `{ payload: {…} }` and flat `{ text, action, style }`; dropping either breaks a call path. `fix` normalizes to `grammar`.
+- **Prevention:** if you touch the handler, keep both shapes and the action normalization.
+
+### L-CORE-04 — Preserve the `data-lexai="true"` guard
+
+- **Root cause / failure boundary:** without it, LexAI's own injected UI re-triggers selection/click handlers.
+- **Prevention:** set `data-lexai="true"` on every injected node; handlers skip `target.closest('[data-lexai="true"]')`.
+
+### L-CORE-05 — Never expose the API key; keep the plaintext fallback
+
+- **Root cause / failure boundary:** the key is a user secret; and legacy installs still have plaintext `apiKey`.
+- **Prevention:** prefer `apiKeyEnc` + `encKey`; never log the key; never send it anywhere except the user's selected provider endpoint; don't drop the plaintext `apiKey` fallback without a migration.
+
+### L-CORE-06 — Provider code is duplicated (`callX` + `callXWithPrompt`)
+
+- **Root cause / failure boundary:** each provider has two near-identical functions; a request-shape change to one silently diverges from the other.
+- **Prevention:** update both until the layer is refactored (TASKS #4). Keep error handling uniform (network → friendly string; `!res.ok` → provider message; empty → explicit message).
+
+### L-CORE-07 — Content script / popup must not call providers
+
+- **Root cause / failure boundary:** CORS and key handling belong in the service worker; a direct provider `fetch` from content/popup leaks the key path and fails CORS.
+- **Prevention:** route everything through `ANALYZE_TEXT` / `COPY_AS` to `background.ts`.
+
+### L-CORE-08 — Version lives in two files
+
+- **Root cause / failure boundary:** manifest version comes from `wxt.config.ts`; `package.json` has its own — they drift and have caused git churn.
+- **Prevention:** bump `version` in **both** `package.json` and `wxt.config.ts` (until T-16 single-sources it). A `v*.*.*` tag triggers the CWS deploy.
+
+### L-CORE-09 — UI is inline styles; Tailwind is inactive
+
+- **Root cause / failure boundary:** Tailwind is installed but WXT PostCSS was never wired; Tailwind classes silently do nothing.
+- **Prevention:** style with inline objects and the existing dark palette; don't add Tailwind classes unless the task is explicitly to wire PostCSS.
+
+## Recording policy
+
+Add a lesson only after a material, evidenced learning signal (correction, unexpected failure, regression, rejected review, proven wrong assumption). Each needs a durable prevention; link a deterministic eval when possible. No secrets, credentials, personal data, or raw transcripts.
+
+## Lesson template
+
+```markdown
+### L-YYYY-MM-DD-NN — [short imperative guardrail]
+- **Status:** active | archived | superseded by [ID]
+- **Trigger / Root cause / Prevention / Evidence / Eval / Owner-review**
+```
+
+## Archive
+
+_Historical lessons move here with their original IDs and a one-line archival reason._